Likely to be Biggs, tho it could be anyone because there were many resistance other than AVALANCHE, just not as active.
What I don't like tho is the Train Graveyard is part of Sector 7 yet all of it was supposedly crushed by the plate. Ofcourse there could be more than one Train Graveyard in Midgar but why play at a distant one than the one down the road?

Fooking 'ell.